It is well known that ethylenically unsaturated monomers readily polymerize when heated and that the polymerization rate increases with increasing temperature. For this reason, the monomers are stabilized by the addition of substances which act as inhibitors or retarders of polymerization, or which block the initiation mechanism leading to the formation of radicals.
Certain ethylenically unsaturated (diolefin) monomers such as butadiene and isoprene polymerize when left in storage tanks and during transportation at temperatures as low as room temperature. This polymerization is initiated by reaction of the diolefin monomer with oxygen present in the monomer containing system. This reaction forms peroxides and free radical species which can further react with the diolefin monomer.
To prevent this polymerization from taking place, diolefin monomer manufacturers frequently add tert-butyl catechol (TBC) as a polymerization inhibitor. However, TBC is a toxic compound and can cause tissue damage when it contacts skin. These characteristics make it desirable to utilize an inhibitor compound that avoids these difficulties as well as being water and caustic extractable so that it could be easily removed from the finished product before use or additional processing.
The present inventor has discovered that dihydroxycinnamic acid and its derivatives effectively inhibit the polymerization of ethylenically unsaturated monomers during storage and transportation. These compounds prove particularly effective in the presence of oxygen and at temperatures up to 100.degree. C.